Energy harvesting with a micro-electro-machanical system (MEMS)

ABSTRACT

Apparatuses and method are described to create an energy harvesting microstructure, referred to herein as a transduction micro-electro mechanical system (T-MEMS). A T-MEMS includes a substrate, a first buckled membrane, the first buckled membrane has a buckling axis and is connected to the substrate. The first buckled membrane further includes a transduction material, a first conductor, the first conductor is applied to a first area of the transduction material; and a second conductor, the second conductor is applied to a second area of the transduction material, wherein electrical charge is harvested from the transduction material when the first buckled membrane is translated along the buckling axis.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates generally to micro-electro-mechanical systems(MEMS) and or micro-opto-electro-mechanical systems (MOEMS), and morespecifically to apparatuses and methods used to harvest energy with aMEMS or MOEMS device.

2. Art Background

Modern society demonstrates an insatiable appetite for energy. Thisappetite for energy spans a wide spectrum of devices and uses such asimplantable devices, e.g., pace makers, hearing aids, etc.; handhelddevices, e.g., telephones, smart phones, tablets, laptop computers,global positioning system (GPS); and large devices that heat and coolbuildings and power automobiles, trucks, busses, building, etc. The listis endless and the need continues to grow with no end in sight. Whilethe need for energy exists across this wide spectrum of devices, oftenenergy is wasted. Wasted energy, if collected, can be used as theprimary source of energy for a device. Thus, a simultaneous need tocollect wasted energy exists. Collection of wasted energy is alsoreferred to as energy harvesting.

A class of materials called ferroelectrics, which includes piezoelectricmaterials, can be used to harvest energy. Ferroelectric materials can becombined with micro-electro-mechanical systems (MEMS) ormicro-optical-electro-mechanical systems (MOEMS) to harvest energy,thereby turning wasted vibrational energy into electrical energy. Asused herein such a device will be referred to as a T-MEMS energyharvester, where “T” refers to the “transduction” nature of the device,whereby electrical energy is generated by an induced strain in atransduction material that produces electrical energy in response to thestrain. The resulting T-MEMS or T-MOEMS energy harvester can be coupledto a rechargeable battery to provide a source of electrical power whichcan power a multitude of devices. The efficiency of existing T-MEMSbased harvesters has been low. This presents a problem.

Bending beam based resonance devices have been made which include apiezoelectric material such as lead zirconate titanate ceramic (PZT) orpolyvinylidene fluoride (PVDF) combined with a mechanical structure thatis designed to subject the piezoelectric material to strain throughvibration. However, due to the finite non-zero stiffness of the supportstructure and the ferroelectric material, a significant amount of energygoes into overcoming the bending stiffness of the combined supportstructure/ferroelectric material to the detriment of the systems abilityto harvest energy. This presents a problem.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. The invention is illustrated by way ofexample in the embodiments and is not limited in the figures of theaccompanying drawings, in which like references indicate similarelements.

FIG. 1A illustrates a cross-sectional view of a multilayermicrostructure, according to embodiments of the invention.

FIG. 1B illustrates a cross-sectional view of a buckled layer within amicrostructure, according to embodiments of the invention.

FIG. 1C illustrates a cross-sectional view of a buckled layer within amicrostructure, according to additional embodiments of the invention.

FIG. 1D illustrates a cross-sectional view of a buckled layer within amicrostructure, according to additional embodiments of the invention.

FIG. 1E illustrates various projected areas of buckling layers within amicrostructure, according to embodiments of the invention.

FIG. 2A illustrates a cross-sectional view of a multilayer bucklinglayer within a microstructure, according to embodiments of the invention

FIG. 2B illustrates contacting a buckled microstructure membrane with apseudo point load, according to embodiments of the invention.

FIG. 2C illustrates a force displacement curve for a multi-zonemicrostructure spring, according to embodiments of the invention.

FIG. 2D illustrates a second stable state of a buckled membranemulti-zone microstructure spring, according to embodiments of theinvention.

FIG. 3A illustrates a cross-sectional view of a buckled multi-layeredmembrane within a microstructure, according to embodiments of theinvention.

FIG. 3B illustrates a force displacement curve for a multi-zonemicrostructure spring, according to additional embodiments of theinvention.

FIG. 4A illustrates a cross-sectional view of an energy harvestingmicrostructure, according to embodiments of the invention.

FIG. 4B illustrates a top view of the energy harvesting microstructureof FIG. 4A, according to additional embodiments of the invention.

FIG. 4C illustrates a cross-sectional view of an energy harvestingmicrostructure with isolated transduction regions, according toadditional embodiments of the invention.

FIG. 4D illustrates interdigitated conductors, according to embodimentsof the invention.

FIG. 5A illustrates an energy harvesting microstructure utilizing d₃₃poling, according to additional embodiments of the invention.

FIG. 5B illustrates the transduction layer of the energy harvestingmicrostructure of FIG. 5A, according to additional embodiments of theinvention.

FIG. 5C illustrates a plurality of transduction regions, according toadditional embodiments of the invention.

FIG. 6A illustrates a force-displacement characteristic of a firstbuckled membrane, according to embodiments of the invention.

FIG. 6B illustrates a force-displacement characteristic of a secondbuckled membrane, according to embodiments of the invention.

FIG. 6C illustrates aligning load/displacement responses of two coupledbuckled membranes to produce a reduced stiffness zone, according toembodiments of the invention.

FIG. 7A illustrates an operating zone of a reduced stiffness elasticmicro-mechanical structure, according to embodiments of the invention.

FIG. 7B illustrates another operating zone of a reduced stiffnesselastic micro-mechanical structure, according to embodiments of theinvention.

FIG. 8 illustrates locating two substrates, according to embodiments ofthe invention.

FIG. 9 illustrates bonding two substrates together, according toembodiments of the invention.

FIG. 10A illustrates a pair of coupled buckled membranes, according toembodiments of the invention.

FIG. 10B illustrates a method for harvesting energy from buckledmembranes according to one embodiment of the invention.

FIG. 10C illustrates charging a battery with energy harvested frombuckled membranes according to one embodiment of the invention.

FIG. 10D illustrates a method for harvesting energy from a buckledmembrane according to one embodiment of the invention

FIG. 11 illustrates a mass captured between a pair of coupled buckledmembranes, according to embodiments of the invention.

FIG. 12A illustrates stops used with a pair of coupled buckled membranesin an exploded view, according to embodiments of the invention.

FIG. 12B illustrates stops used with a pair of coupled buckledmembranes, according to embodiments of the invention.

FIG. 13A illustrates energy harvesting from a pair of coupled buckledmembranes, according to embodiments of the invention.

FIG. 13B illustrates energy harvesting from a pair of coupled buckledmembranes, according to additional embodiments of the invention.

FIG. 14 illustrates a system for harvesting energy, which includes anactivation/control circuit.

FIG. 15 illustrates a method for harvesting energy with buckledmembranes.

DETAILED DESCRIPTION

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings in which like referencesindicate similar elements, and in which is shown by way of illustration,specific embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those of skillin the art to practice the invention. In other instances, well-knowncircuits, structures, and techniques have not been shown in detail inorder not to obscure the understanding of this description. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the invention is defined only by theappended claims.

Apparatuses and methods are described for harvesting energy with amicro-electro-mechanical system which incorporates a transductionmaterial, referred to herein as a T-MEMS. In one or more embodiments aT-MEMS is made with a buckled membrane. In one or more embodiments, astructure with zero stiffness or near zero stiffness is created from apair of buckled membranes on a microstructure to form the T-MEMS. Inother embodiments a T-MEMS is made with a reduced stiffnessmicro-mechanical structure (RSM) created from a pair of buckledmembranes where the stiffness of the composite structure is a fractionof the stiffness of either individual buckled membrane. Eliminating orlowering the mechanical stiffness of the T-MEMS energy harvesterovercomes one of the existing problems with current vibrational energyharvesters.

In various embodiments, a RSM is created from a pair of multi-zonedmicrostructure springs as described in this description of embodiments.As used in this description of embodiments, both zero-stiffnessmicro-mechanical structures and reduced stiffness micro-mechanicalstructures are included in the term RSM. The RSM can be used with atransduction micro-electro-mechanical system (T-MEMS) device and/or atransduction micro-opto-electro-mechanical system (T-MOEMS) device, or auser defined device that can incorporate aspects of both MEMS and MOEMSdevices. Embodiments of the invention are not limited by the applicationof the T-MEMS energy harvester, and those of skill in the art willrecognize that a T-MEMS energy harvester can be used in embodiments ofthe invention with devices that do not yet exist.

In all of the figures contained in this description of embodiments,geometrical elements are presented to illustrate concepts and parts ofvarious microstructures. In so doing, geometrical elements such as:thickness, lengths, displacements, and shapes are shown which are not toscale. These geometrical elements have been chosen to facilitateunderstanding within the confines of the paper on which theillustrations are presented; therefore, relative proportions andabsolute dimensions should not be inferred therefrom. Additionally, inthe figures that follow, a finite horizontal extent of substrates andlayers is shown such that only one or two buckled membranes or two pairof opposing buckled membranes are illustrated in any given figure. Thoseof skill in the art will recognize that the substrates can extend toencompass many buckled membranes on a single substrate. No limitation isintended, and the finite extent of the substrate and buckled membranesshown is intended to preserve clarity in the illustrations.

FIG. 1A illustrates, generally at 100, a cross-sectional view of amultilayered microstructure, according to embodiments of the invention.With reference to FIG. 1A, the multi-layered microstructure 102 has asubstrate 104, with a thickness indicated at 108. The substrate 104 canbe made from a semiconductor material such as silicon. A buckling layeris illustrated at 106, with a thickness indicated at 110. In the viewshown in 100, the buckling layer 106 has not released from the substrate104. The buckling layer 106 can be made of a variety of materials suchas silicon dioxide, silicon nitride, etc. selected such that buckling ispromoted in the final structure after processing.

In one or more embodiments, the buckling layer 106 can be made from aplurality of layers (sub-layers), as described below in the figures thatfollow, or the buckling layer can be a monolayer. An important propertyof the materials selected for the buckling layer and the substrate isthat a compressive stress should be developed in the buckling layerrelative to the substrate. This is accomplished by different methods invarious embodiments, and in some embodiments it is accomplished byarranging for the coefficient of thermal expansion of the buckling layerto be less than the coefficient of thermal expansion of the substrate.Or in the case of a multilayered buckling layer, at least one layer ofthe sub-layers has a coefficient of thermal expansion that is less thanthe coefficient of thermal expansion of the substrate.

The buckling layer and its sub-layers can be prepared from an inorganicmaterial such as silicon, silicon dioxide in any of its forms(monocrystalline, polycrystalline, microcrystalline or amorphous)silicon germanium, silicon nitride, silicon carbide, diamond, carbon,titanium nitrite, a metallic material such as for example, titanium,gold, copper, aluminum, or alloys thereof, an organic material such asfor example, a photosensitive resin such as benzocyclobutene (BCB),parylene, polydimethylsiloxane (PDMS), polyethylene terephthalate (PET),poly-para-xylylene, etc. The buckling layer can be made by any knownmeans or by yet unknown means, the exact way in which the buckling layeris made does not limit embodiments of the invention.

Some non-limiting examples of how a buckling layer can be formed bymeans known to those of skill in the art are, but are not limited to;thin film deposition, film lamination, spin definition, spraying organicmaterials, ion implantation, etc. Thin film deposition can be performedby chemical vapor deposition (CVD), low pressure chemical vapordeposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD),physical vapor deposition (PVD), electrolysis, epitaxy, thermaloxidation, and vacuum deposition.

FIG. 1B illustrates, generally at 120, a cross-sectional view of abuckled layer within a microstructure, according to embodiments of theinvention. With reference to FIG. 1B, a microstructure 122 has asubstrate 123 and a buckling layer 128. A region having a widthindicated by 124 is patterned and etched to a depth shown as 126.Removal of substrate 123, from the region 124, causes buckling layer 128to release from a flat planar orientation as illustrated by 106 (FIG.1A) to the curved shape as illustrated by 128 (FIG. 1B). The bucklinglayer 128 develops a maximum displaced height indicated by 134. Thebuckling layer 128 has a horizontal extent indicated by 132. Note thatthe horizontal extent 132 can be less than the extent of the region 124.The relationship between the horizontal extent 132 and the region 124depends on the thickness of the buckling layer, the material properties,the magnitude of the compressive stress, and the dimensions and geometryof the projected shape of a buckling layer. In one non-limiting example,a horizontal extent 132 (850 μm) was 85% of the length of the region 124(1000 μm). A top view 180 of the buckling layer 128 is illustrated in180 (FIG. 1E) for a variety of projected shapes.

FIG. 1C illustrates, generally at 150, a cross-sectional view of abuckled layer (buckled membrane) within a microstructure, according toadditional embodiments of the invention. With reference to FIG. 1C,buckling of a layer can be obtained by localized ion implantation intothe layer. The substrate 154 is removed in a region 158 from behind thebuckling layer 156. Ions implanted during ion implantation induce acompressive stress in the buckling layer 156. The compressive stressinduces buckling, which results in a release of a buckling layer 156into a curved shape that functions as a multi-zoned microstructurespring. The magnitude of the compressive stress induced in the layerdepends on the dose selected and the types of ions used. Heat treatmentcan be applied at approximately 200 degrees C. to trigger release of thebuckling layer. 156 shows the buckling layer after release into a curvedshape. Note that the buckling layer 156 and the substrate 154 can be thesame material; different coefficients of thermal expansion within layersis not required in order to release a buckling layer via ionimplantation. Thus, in the example of FIG. 1C both the substrate and thebuckling layer can be made from silicon.

Ion implantation can also be used to release a buckling layer when thebuckling layer has a coefficient of thermal expansion that is largerthan the coefficient of thermal expansion of the substrate. Formation ofa layer with a coefficient of thermal expansion which was larger thanthe coefficient of thermal expansion of the substrate would not resultin release of the buckling layer since the buckling layer would be in astate of tension. Ion implantation is used to overcome the state oftension and to create a state of compression, which will result inrelease of the buckling layer.

Note that the state of tension or compression within a buckling layer ofa microstructure depends on, in general: the coefficients of thermalexpansion of the materials (e.g., individual; layers of themicrostructure), the formation temperature of the microstructure, andthe operating temperature of the microstructure. For example, a bucklinglayer that is in a state of tension at 20 degrees Centigrade will buckleat a temperature above 20 degrees Centigrade (e.g., 200 degreesCentigrade) since the buckling layer will expand more than the substrate(assuming a coefficient of thermal expansion of the buckling layer islarger than a coefficient of thermal expansion of the substrate).Therefore, materials, coefficients of thermal expansion, operatingtemperatures, and process steps, e.g., ion implantation are consideredwhen buckling is desired within a microstructure.

Many variations in microstructures can exist and many differentmaterials can be used for the substrate and buckling layers. Therefore,depending on the material used for the substrate and the material usedfor the desired buckling layer, the species to be implanted via ionimplantation will be selected to achieve appropriate compressive stresswithin the material's lattice structure.

An example of a buckling layer having a coefficient of thermal expansionthat is greater than the coefficient of thermal expansion of a substrateis a buckling layer made from silicon nitride Si₃N₄, having acoefficient of thermal expansion of 3.4×10-6 mm/K which can be releasedfrom a substrate of silicon having a coefficient of thermal expansion of2.6×10-6 mm/K via ion implantation. Similarly, a buckling layer can bemade from alumina Al₂O₃ and can be released from a silicon substrate.

FIG. 1D illustrates, generally at 160, a cross-sectional view of abuckled layer within a microstructure 162, according to additionalembodiments of the invention. Referring to FIG. 1D, a buckling layer 166is illustrated that has reduced adhesion to a substrate 164 over aregion 168. Outside of the region 168, the buckling layer 166 hasadhesion sufficient to overcome any differential compressive stressresiding within the buckling layer. Adhesion between the buckling layer166 and 164 over the area 168 is too low to prevent buckling.

Referring to 170 in FIG. 1D, release of a buckling layer can beaccomplished by application of a first layer that has limited adhesionwith the substrate 174, such as a layer 179. In various embodiments, thelayer 179 is made from an organic material which provides less adhesionthan the adhesion obtained between the buckling layer 176 and thesubstrate 174. In various embodiments, the layer 179 can be made frompolyethylene terephthalate, parylene, benzocyclobutene, etc. Forexample, a sub-layer of parylene can be deposited by gas spraying on thesubstrate, patterned by a photolithography step to limit application tothe area where release is desired, i.e., 178. Subsequent application ofthe buckling layer 176 results in limited adhesion under the area 178.Release of the buckling layer can be accomplished by any of the methodspreviously described, resulting in the buckled membrane 176.

Adhesion between a buckling layer 166 and the substrate 164 can bedecreased by a process step(s) in the manufacture of the microstructurefor example by an application or a deposition of an intermediate layer(not shown in FIG. 1D at 160), such as by application of liquids e.g., aphotosensitive resin such as Unity 2203P from Promerus, LLC.

Depending on the substrate and intended use of the microstructure,buckling can be induced during deposition of a second sub-layer on topof a first sub-layer where the second sub-layer has a higher coefficientof thermal expansion than the coefficient of thermal expansion of thefirst sub-layer. A vacuum heat treatment at 300 degrees C. for severalminutes can facilitate release of the buckling layer.

Alternatively, for a liquid sub-layer of propylene carbonate or water,degradation of adhesion can occur from evaporation at temperaturesaround 100 degrees C., resulting in release of the buckling layer.

FIG. 1E illustrates, generally at 180, various projected areas ofbuckling layers within a microstructure, according to embodiments of theinvention. With reference to FIG. 1E, a released buckled shape can haveany type of projected area. The shape of the projected area depends onthe patterning that was employed in order to release a buckling layerover an area. A rectangular shape is indicated at 182, a round shape isindicated at 184, and an oval shape is indicated at 186. A triangularshape is indicated at 188, a square shape is indicated at 190, and ageneral user defined shape is indicated at 192.

FIG. 2A illustrates, generally at 200, a cross-sectional view of amultilayer buckling layer within a microstructure, according toembodiments of the invention. In one embodiment, a silicon-on-insulator(SOI) wafer 202 was patterned to create a matrix of buckled membranes(only one is shown in the figure for clarity). In this example, anN-type silicon substrate 204 has a thickness of 400 micrometer (μm)indicated at 216. A silicon dioxide layer 206, having a thickness 208 of2 μm was thermally grown on a top layer 210 also referred to as a devicelayer having a thickness 212 of 5 μm and made from the same material asthe substrate 204 and then sandwiched between the substrate 204 anddevice layer 210. The bottom of the substrate was patterned over an area214 and etched to a depth of 400 μm, thereby removing the substrate frombeneath the silicon dioxide layer 206.

Various methods can be used to etch the substrate 204. For example, inone embodiment, a layer of silicon dioxide (not shown for clarity) wasdeposited on the bottom of the wafer 202 followed by a layer of photoresist (also not shown for clarity). The photo resist was thenlithographically patterned and developed using standard micromachiningtechniques. A Buffered Oxide Etch was then used to etch through thesilicon dioxide layer and then Deep Reactive Ion Etching (DRIE) was usedto remove silicon selectively from beneath the silicon dioxide layer206. Other techniques can be used such as an anisotropic wet etch usingTetramethylammonium Hydroxide (TMAH). Other micromachining techniquescan be used; embodiments of the invention are not limited by the choiceof micromachining techniques.

After removal of the substrate 204, over the area 214, the silicondioxide/device layer 206/210 releases to form a buckling layer 205 withtwo sub-layers. This is the first of two stable states for the bucklinglayer. The other or second stable state is referred to as a “poppedthrough” position and is as illustrated below in FIG. 2D. In thisexample, the resulting buckled membrane 205 had a projected area whichwas square in shape with dimensions of 1 millimeter (mm) on edge. Aresulting maximum deflected height 218 is approximately 15 μm.

FIG. 2B illustrates, generally at 220, contacting a buckledmicrostructure membrane with a pseudo point load, according toembodiments of the invention. Referring to FIG. 2B, a calibrated microNewton (μN) force sensor 222 is used to apply a pseudo point load to thebuckled membrane 205 over a contact area 226. In the experimentresulting in the measurement shown below in FIG. 2C, a force sensor fromFemtoTools model FT-S270 was used. This force sensor, mounted to apiezoelectric actuator with a translation range of 20 μm, is capable ofapplying loads in the micro Newton range (μN). Force sensor 222 has anactuator 224. The actuator 224 has a contact pad 228 which applies theload to the buckled membrane 205 over the contact area 226. The contactarea 226 is the portion of the buckled membrane 205 that is directlyunder the contact pad 228. The contact area 226 is much smaller than thesurface area of the buckled membrane 205. The dimensions of the contactpad are 50 μm by 50 μm. It is important to note that the force sensor222 and the actuator 224 are used to apply a controlled displacement(along with a corresponding application of force) to the buckledmembrane 205. The controlled displacement is used to determine a loadthe buckled membrane applies in response to the controlled displacement.

During the measurement procedure that resulted in the measurementpresented in FIG. 2C, the force sensor actuator 224 (FIG. 2B) applies aload to the buckled membrane 205, which causes the buckled membrane 205to translate in the direction indicated by an arrow 231. One suchsubsequent position associated with the translation of the actuator 224and the buckled membrane 205 is illustrated by the dashed linesindicating the displaced position of the actuator and membrane and apoint 230. The point 230 shows the location of the actuator tip-buckledmembrane interface at one point during the measurement procedure.Following the procedure of applying an increasing displacement to thebuckled membrane 205 the measured force verses displacement curvereveals a multi-zoned spring as illustrated in FIG. 2C. This multi-zonedspring has a force-displacement characteristic which corresponds to astiffness profile which is initially positive, transitions through zeroand then becomes negative. Further application of force results in alarge deflection of the buckled membrane to a second stable state withcurvature that is opposite that of the first stable state. The secondstable state is illustrated below in conjunction with FIG. 2D.

FIG. 2C illustrates, generally at 240, a force-displacementcharacteristic for the multi-zone microstructure spring of FIG. 2A andFIG. 2B, and a corresponding stiffness-displacement relationship,generally at 260, according to embodiments of the invention. Withreference to FIGS. 2C and 240, force is plotted on the vertical axis 242in micro Newtons (μN) and displacement is plotted on the horizontal axis244 in nanometers. A corresponding stiffness-displacement relationshipis plotted in 260 with stiffness plotted on the vertical axis 262 withunits of kiloNewtons/meter and displacement plotted on the horizontalaxis 264 with units of nanometers. The force-displacement characteristic240 reveals a multi-zone microstructure spring, characterized by threezones.

A first zone, indicated by 248 and 268, the positive stiffness zone, ischaracterized by increasing force with increasing displacement of thebuckled membrane. A second zone, indicated by, 246 and 266, the zerostiffness zone, is characterized by a constant force-displacementcondition. A third zone 250 and 270, the negative stiffness zone, ischaracterized by decreasing force with increasing displacement. As usedin this description of embodiments, the term “zone” can imply a point ora region. The extent or lack of extent of a “zone” is determined by theparticular physical structure. As such, in some cases, the zerostiffness zone will exist as a point. No limitation is implied by theuse of the term “zone” to characterize the physics of the multi-zonemicrostructure spring.

When the buckled membrane passes a displacement illustrated at 254, thebuckled membrane quickly translates away from the force sensor actuator224 to assume its second stable state as shown generally at 270 in FIG.2D at 262.

The methods described above provide an ability to characterize thephysical behavior of a MEMS structure or a MOEMS structure which issuperior to exposing the entire surface of such structures to a uniformload through for example pressure exerted by a fluid or a gas. Oncecharacterized, the resulting membrane stiffness together with a knownenvironmental acceleration level are used to determine the size of asprung mass (attached to the membrane) needed to cause the membrane totransition from the first popped through state to the other in theenergy harvester.

In the case of a membrane having a stiffness similar to the one measuredin FIG. 2C, in one embodiment, an acceleration level of 9.8 m/sec^2(typical of levels experienced during walking) and a sprung mass of 0.2grams are used to cause the buckled membrane to transition between thetwo stable states.

Note that whenever particular values are given, they are given by way ofillustration only and are not meant to limit embodiments of theinvention. Those of skill in the art will recognize that the dimensionsand composition of the membrane can be changed, resulting in a differentsize of the sprung mass. Similarly, the environmental accelerationlevels can be changed which can result in a different desired sprungmass.

FIG. 3A illustrates, generally at 300, a cross-sectional view of abuckled multi-layered membrane within a microstructure, according toembodiments of the invention. Similar to the microstructure of FIG. 2A,in one embodiment, a silicon-on-insulator (SOI) wafer 302 was patternedto create a matrix of buckled membranes. In this example, an N-typesilicon substrate 304 has a thickness of 400 micrometer (μm) indicatedat 320. A silicon dioxide layer 306, having a thickness 308 of 2 μm wasthermally grown and sandwiched between the substrate 304 and a layer310. The layer 310 has a thickness 312 of 5 μm and is made from the samematerial as the substrate 204 (FIGS. 2A, 2B and 2D). Next a 500 angstromthick titanium layer 324 was evaporated to the silicon layer 310. A 3000angstrom thick gold layer was evaporated on top of the titanium layer.The bottom of the substrate was patterned over an area 318 and etched toa depth of 400 μm; thereby removing the substrate from beneath thesilicon dioxide layer 306.

Similar to the etching applied to the microstructure of FIG. 2A, variousmethods can be used to etch the substrate 304, prior to release of abuckling layer 326. For example, in one embodiment, a layer of silicondioxide (not shown for clarity) was deposited on the bottom of the wafer302 followed by a layer of photo resist (also not shown for clarity).The photo resist was then lithographically patterned and developed usingstandard micromachining techniques. A Buffered Oxide etch was then usedto etch through the silicon dioxide layer and then Deep Reactive IonEtching (DRIE) was used to remove silicon selectively from beneath thesilicon dioxide layer 306. Other techniques can be used such as ananisotropic wet etch using Tetramethylammonium Hydroxide (TMAH). Othermicromachining techniques can be used as well; embodiments of theinvention are not limited by the choice of micromachining techniques.

After removal of the substrate 304, over the area 318, the bucklinglayer 326 (which includes layers 306, 310, 324, and 314) releases toform a buckling layer 326 having four sub-layers. In this example, theresulting buckled membrane 326 has a projected area which was square inshape with dimensions of 1 millimeter (mm) on edge, with a predominantlycircular curved shape buckling out of the horizontal plane of thesubstrate. A resulting maximum deflected height 322 is approximately 10μm.

During the measurement procedure that resulted in FIG. 3B, the forcesensor actuator 224 (shown in FIG. 2B but not shown in FIG. 3A forclarity) applies a load to the buckled membrane 326 (FIG. 3A) whichcauses the membrane to translate in the direction indicated by an arrow327 (FIG. 3A). As described above in conjunction with FIG. 2B, theprocedure results in force and displacement being recorded as thebuckled membrane is translated by application of the pseudo point loadapplied by the force transducer actuator. Subsequent positionsassociated with the translation of the buckled membrane and the forcetransducer actuator are not shown for clarity. The procedure applied tothe buckled membrane 326 reveals a multi-zoned spring as illustrated inFIG. 3B. This multi-zoned spring has a force-displacement characteristicwhich corresponds to a stiffness profile which is initially positive,transitions to zero and then becomes negative. Further application offorce results in a large deflection of the buckled membrane to a secondstable state with curvature that is opposite that of the first stablestate.

FIG. 3B illustrates, generally at 330, a force-displacementcharacteristic for the multi-zone microstructure spring of FIG. 3A, anda corresponding stiffness-displacement relationship, generally at 360,according to embodiments of the invention. With reference to FIG. 3B at330, force is plotted on the vertical axis 332 with units of microNewtons (μN) and displacement is plotted on the horizontal axis 334 withunits of nanometers. A corresponding stiffness-displacement relationshipis plotted in 360 with stiffness plotted on the vertical axis 362 withunits of kiloNewtons/meter and displacement plotted on the horizontalaxis 364 with units of nanometers. The force-displacement characteristic330 reveals a multi-zone microstructure spring characterized by threezones.

A first zone indicated by 338 and 368, the positive stiffness zone, ismarked by increasing force with increasing displacement of the buckledmembrane. A second zone indicated by 336 and 366, the zero stiffnesszone, is characterized by a constant force-displacement condition. Athird zone 340 and 370, the negative stiffness zone, is characterized bydecreasing force with increasing displacement. The zero stiffness zone336/366 can be a point or a region. No limitation is implied by the useof the term “zone” to characterize the physics of the microstructurespring.

When the displacement of the buckled membrane approaches the end of thenegative stiffness zone, as indicated by 344, the buckled membranequickly translates away from the force sensor actuator 224 to assume itssecond stable state which is qualitatively shown at 262 in FIG. 2D.

The addition of the metal layers, i.e., 500 angstroms of titanium 324and 3000 angstroms of gold 314 has increased the linearity of both thepositive stiffness zone and the negative stiffness zone as illustratedby the measurements presented in FIG. 3B. In various embodiments, asubstantially linearized negative stiffness spring is obtained from abuckled membrane within a microstructure. Such devices have three zonesfor operation, which are selectable based on an initial displacementfrom a rest position of the buckled membrane.

In various embodiments, the buckled membranes described in the figuresabove are configured with one or more transduction layers or regions ofmaterial to create an energy harvesting microstructure. FIG. 4Aillustrates, generally at 400, a cross-sectional view of an energyharvesting microstructure, according to embodiments of the invention.With reference to FIG. 4A, a multi-layered microstructure is providedfor use as a transduction micro electro-mechanical system (T-MEMS) 401.T-MEMS 401 contains a substrate 402 and buckled membranes 414 a and 416a. In one or more embodiments, the substrate 402 can be an N-typesilicon substrate. Buckled membranes are released from substrate 402over regions 422 and 424 by means described above in the precedingfigures.

A first conductive layer 406 is deposited on the substrate 402 prior toetching substrate from regions 404 a and 404 b. Alternatively, a layerof silicon dioxide is deposited on substrate 404 before application ofthe conductive layer 406. In some embodiments a conductive layer isapplied from a film of LaNiO3 as described in the figures that follow.The first conductive layer 406 can be made out of doped poly-silicon ora metal layer can be deposited to provide the first conductive layer. Atransduction material or layer is shown at 410. The material used forthe transduction layer produces a voltage in response to induced strain.The transduction material can be any material that produces a voltage inresponse to strain.

In various embodiments, the transduction layer 410 can be made from aferroelectric material. Ferroelectric materials include the class ofmaterials know as piezo-electric materials. Some examples offerroelectric (and as well piezo-electric) materials are lead zirconatetitanate ceramic (PZT), polyvinylidene fluoride (PVDF), polyimide,aluminum nitride (AlN), zinc oxide (ZnO), etc. Embodiments of theinvention are not limited by the choice of transduction material andtransduction materials that are yet unknown can be used in embodimentsof the invention.

A second conductive layer 408 is in contact with the transduction layer410. The second conductive layer 408 can made from doped poly-silicon, ametal layer, an LiNiO3 film, etc. An optional layer 412 can be added tothe second conductive layer 408. The optional layer 412 can be aninsulating layer made from silicon dioxide or other material.

After the transduction layer 410 is applied; with the first conductor406 and the second conductor 408, in the case of ferroelectricmaterials, the transduction material 410 is poled by application of anelectric field using the first conductive layer 406 and the secondconductive layer 408 at a temperature above the Curie temperature forthe ferroelectric material. Subsequent to poling and cooling thematerial will exhibit the piezoelectric effect: application of anelectric field applied to the material will produce an expansion orcontraction of the ferroelectric material and strain of the materialwill produce an electric potential across the material, currenttherefrom can be harvested, resulting in the “harvesting of energy.”

The constitutive equations for a piezoelectric material are directiondependent; the material will produce different responses depending onthe poling axis and the excitation axis. Described herein for the caseof a membrane are two axes, the in-plane axis of the membrane and theaxis that is perpendicular to the in-plane axis, which is referred toherein as the thickness axis or thickness direction. For the sake ofdiscussion and convention, two modes are important for a membrane, thed₃₃ mode and the d₃₁ mode. In the d₃₃ mode, the material is poled in the3 direction (430) and the strain is applied in the 3 direction (430),the in-plane axis. In the d₃₁ mode, the material is poled in the 1direction (431) and the material is strained in the 3 direction (430).The 3 direction (430) is the in-plane axis of the membrane and the 1direction (431) is perpendicular to the plane of the membrane. Thus, thedescription given for FIG. 4A above describes d₃₁ poling. d₃₃ poling isdescribed below in FIG. 4D, and FIG. 5A-5C.

In one or more embodiments, lead zirconate titanate ceramic (PZT) isused for the transduction material that is used in a transduction layeror region of the transduction electro-mechanical system (T-MEMS). A PZTlayer can be created in various ways as is known to those of skill inthe art. One such way, given herein as a non-limiting example startswith a PZT solution created with lead acetate trihydrate, titanium-IVisopropoxide, and zirconium-IV propoxide as precursors and2-methoxyethanol as a solvent. In order to maximize the efficiency of aT-MEMS device it is desirable to maximize the piezoelectric constants ofthe transduction region. In the case of a PZT material this occurs whenthe ratio of Zr to Ti is in the ratio of 52/48.

A sol-gel based 52/48 PZT thin film begins with lead acetate trihydrateand 2-methoxyethanol dehydrated at 120 degrees Centigrade. Thezirconium-IV propoxide, titanium-IV propoxide, and 2-methoxyethanol aremixed and stirred at 25 degrees Centigrade and the result is combinedwith dehydrated lead acetate trihydrate. This mixture is refluxed forthree hours at 120 degrees Centigrade and by-products are distilled offat 120 degrees Centigrade. Formamide, 4% by volume, is added as a dryingcontrol agent after refluxing. The resulting PZT solution has aconcentration of 0.4 M (molar). Other concentrations can be created andused to create the PZT film.

In one embodiment, the starting substrate consists of a silicon dioxidelayer deposited on a silicon wafer, known as SOI to those of skill inthe art; other wafers can also be used. The thickness of the silicondioxide layer can vary and in some cases it is desirable for this layerto be between 0.5 μm (micrometers) and 2.0 μm thick. In other cases thesilicon dioxide layer can be tens of micrometers in thickness.

The silicon dioxide layer is followed by application of a 200 A(angstrom) thick layer of Ta (Tantalum) or Ti (Titanium) adhesion layerand then an 800 to 2000 A layer of Pt (Platinum). The Pt layer becomesone the electrodes (conductors) needed for the PZT layer. Ta, Ti, and Ptcan be applied by sputtering or by other standard micro-machiningtechniques.

The PZT film is built-up by repetition of processing steps as follows:(1) PZT solution is deposited onto the Pt layer; (2) the substrate isspun for 30 seconds at 2500 RPM; (3) the wafer is placed on a hot platefor pyrolysis at 350 degree Centigrade for 2 minutes; (4) after everyfour deposition and pyrolysis cycles (steps 1-3 equals one cycle) acrystallization process is performed which is performed in a rapidthermal annealing (RTA) furnace at 700 degrees Centigrade for 30seconds.

The PZT film is built-up on the substrate by successive application ofsteps 1-3 at a rate of approximately 0.05 μm/cycle. PZT films can begrown to various desired thicknesses such as 0.25 μm-tens of μm.Embodiments of the invention are not limited by the thickness of the PZTlayer.

After the PZT layer has grown to the desired thickness, a secondconductor layer is applied on top of the PZT. The second conductor canbe a layer of metal such as Pt on the order of 800 to 2000 A thick.Alternatively the second conductor can be made from doped Polysilicon.The second conductor can be deposited over photoresist for later removalto obtain a desired pattern for the second conductor.

Alternatively, a conductive layer can be created by depositing a LaNiO3thin film on p+ silicon, the LiNiO3 then acts as a conductive layer. ALaNiO3 layer can be derived from a metal organic decomposition technique(MOD). Lanthanum nitrate and nickel acetate are used as startingmaterials, and acetic acid and water are used as the solvents. Nickelacetate is dissolved in acetic acid at room temperature and lanthanumnitrate is dissolved in water. The two solutions are mixed together andformamide is added to prevent cracking of the film during pyrolysis(water/formamide volumetric ratio of 6:1). Acetic acid is added toachieve a final concentration of approximately 0.3M.

The LaNiO3 film is prepared by spinning the solution onto the siliconsubstrate at several thousand RPM for 30 seconds. Following spinning,annealing is performed at 300 degrees Centigrade for 60 seconds. Thefilm is then annealed at 700 degrees Centigrade for 60 seconds usingrapid thermal annealing (RTA). The deposition and annealing processesare repeated until the desired LaNiO3 film is built-up to the desiredthickness. Typical LaNiO3 film thicknesses are in the range tenths ofmicrometers with 0.3 μm being a typical thickness.

The PZT film can be poled by application of an electric field across thetwo conductors. Poling field strength is in the kilovolt/cm range, with20-80 kilovolts/cm being a typical range. Poling temperatures range fromroom temperature to above the Curie temperature for the transductionmaterial. Poling times are on the order of minutes. When the PZT film ispoled at elevated temperature the resulting piezoelectric coefficientsare at a higher value. In one embodiment, a PZT film is poled for 15minutes at 50 kV/cm at room temperature and in another embodiment, thePZT film is poled at 200 degrees Centigrade for 15 minutes resulting inan increase in the value of the piezoelectric constants.

In other embodiments, polyimide is used for the transduction material.Polyimide is a flexible material that can maintain its piezoelectricproperties up to 150 degrees centigrade. A process for applying layersor regions of polyimide consists of spin-casting onto a silicon wafer. Apolyimide precursor solution is prepared with concentrations of 15% or20% by weight of polyimide being typical. The surface on which thepolyimide film is to be applied must be dry therefore, a short 15second, 90 degree centigrade pre-bake is used to remove moisture fromthe wafer. Nominal spin speeds of 4000 revolutions per minute are usedand application of the polyimide precursor material at 15% and 20%concentrations results in a film thickness of 1.3 um and 4.0 umrespectively. Successive applications of the precursor material can beemployed to build up a thicker film if desired. After the polyimide filmhas been deposited the solvents are driven off with a 24 hour bake at 90degrees centigrade. A conductive layer can be applied by sputtering ametal layer thereon such as aluminum. The aluminum electrode can bepatterned with standard lithography and etching. The polyimide can beetched in an O₂ plasma. Alternatively, a directional reactive ion etchcan be used on the polyimide. Additional details on using a polyimidelayer in a micro-machined structure can be found in PiezoelectricPolyimide MEMS Process, National Institute of Aerospace, NASA LangleyResearch Center Hampton, Va. 23681.

In various embodiments, other materials such as aluminum nitride (AlN)are used for the transduction layer. AlN films can be sputtered onto asilicon substrate and processed with standard micromachining techniques.Alternatively, in other embodiments, Zinc Oxide (ZnO) is used as theconduction material. ZnO is applied to a substrate by various techniquessuch as sol-gel technique, chemical vapor deposition, (CVD), pulse laserdeposition (PLD), spray pyrolysis, magnetron sputtering, etc.

The buckled membrane 414 a is shown in a first stable state. A secondstable state or popped through state is indicated at 414 b. A mass 418is attached to the buckled membrane 414 a. Vibrations from any source oforigin (and the resulting accelerations therefrom) will cause the mass418 to apply a force to the buckled membrane 414 a, thereby causing thebuckled membrane 414 a to travel from one stable state to the othergenerating energy from the strain imparted to the transduction layer 410over regions 422 and 424 as the membrane flexes. Optional stops can beprovided, as described below in conjunction with FIGS. 12A and 12B tokeep the buckled membrane vibrating in a zone of desired stiffness, asdescribed below in conjunction with FIGS. 7A and 7B.

Similarly, buckled membrane 416 a is shown in a first stable state. Asecond stable state for buckled membrane 416 a is 416 b. A mass 420 isattached to the buckled membrane 416 a. Vibrations from any source oforigin (and the resulting accelerations therefrom) will cause the mass420 to apply a force to the buckled membrane 416 a, thereby causing thebuckled membrane 416 a to travel from one stable state to the othergenerating energy as it does so. The energy generated by the movingmembranes is harvested and transferred to an energy storage device suchas a capacitor or rechargeable battery using appropriate circuitelements not shown.

An extent of the buckled membrane 414 a is shown at 422. Similarly, anextent of the buckled membrane 416 a is shown at 424.

FIG. 4B illustrates, generally at 450, a top view of the energyharvesting microstructure 402 of FIG. 4A, according to additionalembodiments of the invention. With reference to FIG. 4B, a top view 454of the first buckled membrane 414 a is shown and a top view 458 of thesecond buckled membrane 416 a is shown. Any number of additional buckledmembranes can exist on the T-MEMS of FIG. 4A and FIG. 4B. Two optionaladditional buckled membranes are shown at 460 and 462.

The plurality of buckled membranes shown in FIG. 4A (414 a and 416 a,etc.) are configured with common first and second conductors whichcombine, in parallel, the electrical energy outputs of the buckledmembranes within the conductors. In some embodiments, it is desirable tobe able to access the electrical outputs of buckled membranesindividually without combining them in the conductor; such a mechanismis shown in FIG. 4C.

FIG. 4C illustrates a cross-sectional view, generally at 470, of anenergy harvesting microstructure with isolated transduction regions,according to additional embodiments of the invention. With reference toFIG. 4C, the transduction layer 410 from FIG. 4A is partitioned intomultiple transduction regions, i.e., a transduction region 410 a and atransduction region 410 b. Insulating material 411 is used in betweenthe transduction region 410 a and the transduction region 410 b.Insulating material 411 can be made, in one example, from silicondioxide (SiO₂).

Similarly, the first conductor 406 from FIG. 4A is divided intoindividual conductors 406 a and 406 b, thereby separating electricallythe first conductor into areas specific to the transduction region 410 aand the transduction region 410 b. Insulating areas 407 within the layerof the first conductor can be made in one embodiment from SiO2. Thesecond conductor 408 FIG. 4A is divided into individual conductors 408 aand 408 b, thereby separating electrically the second conductor intoareas specific to the transduction region 410 a and the transductionregion 410 b. Insulating areas 409 within the layer of the secondconductor can be made in one embodiment from SiO2. Appropriateconnection to the conductors 406 a and 408 a allows the energy fromtransduction region 410 a to be channeled separately from the energyfrom conductors 406 b and 408 b. Energy from the transduction region 410b is accessed from the conductors 406 b and 408 b.

FIG. 4D illustrates, generally at 480, interdigitated conductors,according to embodiments of the invention. With reference to FIG. 4D, amicrostructure 482 has a surface 484. The surface 484 can be in variousembodiments, a layer or region of transduction material.

A first conductor 486 includes a plurality of semicircular portions 488.A second conductor 490 includes a plurality of semicircular portions492. Semicircular portions 488 and semicircular portions 492 areseparated by a distance d (indicated at 494) and are referred to asinterdigitated. In various embodiments, interdigitated conductors490/492 and 486/488 are located on the same surface 484, which puts bothconductors on the same side of the transductive layer or region.

Interdigitated conductors can be arranged in a variety of geometriessuch as circular, oval, square, etc. When an electrical field is appliedbetween the first conductor 486/488 and the second conductor 490/492 thetransduction layer or region is poled in the 3 direction (see FIG. 4A at430), which utilizes the d₃₃ direction of the transduction layer orregion.

The two interdigitated conductors 486/488 and 490/492 are applied bymeans described herein and by means know to those of skill in the art.

FIG. 5A illustrates an energy harvesting microstructure utilizing d₃₃poling, generally at 500, according to additional embodiments of theinvention. With reference to FIG. 5A, a multi-layered microstructure isprovided for use as a transduction micro-electro-mechanical system(T-MEMS) 501. T-MEMS 501 contains a substrate 502 and buckled membranes514 a and 516 a. In one or more embodiments, the substrate 502 can be anN-type silicon substrate. Two energy producing membranes are shown in501; however any number of energy producing membranes can be configuredinto 501. A first transduction region 510 a is bounded by a firstconductor 512 a and a second conductor 508. A second transduction region510 b is bounded by a first conductor 512 b and the second conductor508.

Using standard micromachining techniques, transduction material isapplied in a layer, which is then patterned and etched to provide thefirst transduction region 510 a and the second transduction region 510b. In the case of ferroelectric materials, the transduction region 510 ais poled in the d₃₃ direction 430 by application of an electric fieldbetween the first conductor 512 a and the second conductor 508 at atemperature above the Curie temperature for the ferroelectric material.Subsequent to poling and cooling the material will exhibit thepiezoelectric effect: application of an electric field applied to thematerial will produce an expansion or contraction of the ferroelectricmaterial and strain of the material will produce an electric potentialacross the material, electrical charge therefrom can be harvested,resulting in the “harvesting of electrical energy.” In like manner, thesecond transduction region 510 b is poled by application of an electricbetween the first conductor 512 b and the second conductor 508. Notethat the electric field is applied in the “3” direction as indicated at430 which indicates the in-plane axis of the membrane. When the membranebuckles and flexes from one stable state to another the strain developedin the membrane is also in the “3” direction.

The T-MEMS 501 can have one or more optional layers as shown in FIG. 5A.In one or more embodiments, an optional insulating layer 507 can beapplied to the substrate 502. In one or more embodiments, the substrate502 can be made from silicon and the insulating layer 507 can be madefrom silicon dioxide. Also, in one or more embodiments, an optionalinsulating layer 518 can be applied to the transduction/conductor layer.The optional insulating layer 518 is then patterned and etched to revealthe first conductor 512 a and the first conductor 512 b. The secondconductor can be accessed through appropriate patterning and etching aswell.

In various embodiments, the buckled membranes 514 a and 516 a arereleased from the substrate 501 by etching or other appropriatetechnique from the substrate 502 in the area 504 and 506 over theregions 522 and 524 as described above in conjunction with the precedingfigures. The first buckled membrane 514 a can travel as shown from afirst stable state as indicated by the dashed line of 514 a to a secondstable state, as indicated by a dashed line of 514 b. Similarly, thesecond buckled membrane 516 b can travel as shown from a first stablestate as indicated by the dashed line of 516 b to a second stable state,as indicated by a dashed line of 516 b.

The first conductors 512 a, 512 b, and the second conductor 508 can bemade out of doped poly-silicon or a metal such as gold, silver,platinum, etc. or other film such as LaNiO3 can be deposited to providethe conductors. The conductors can be formed by patterning and etching aconductive layer applied either before or after the transductionmaterial is applied.

Just as in the case of d₃₁ poling, described above, the material usedfor the transduction regions produces a voltage in response to inducedstrain. The transduction material can be any material that produces avoltage in response to strain such as PZT, PVDF, etc. In variousembodiments, the transduction regions 510 a and 510 b can be made from aferroelectric material. Ferroelectric materials include the class ofmaterials know as piezo-electric materials. Some examples offerroelectric (and as well piezo-electric) materials are lead zirconatetitanate ceramic (PZT) or polyvinylidene fluoride (PVDF). Embodiments ofthe invention are not limited by the choice of transduction material andtransduction materials that are yet unknown can be used in embodimentsof the invention.

In some embodiments, it can be desirable to configure each buckledmembrane so that they are electrically isolated from each other. In sucha case, the second conductor is segregated into separate conductors thatonly have contact with one transduction region. The second conductor ispatterned as is shown by optional insulating material 518. Insulatingmaterial 518 can be made from silicon dioxide in one or moreembodiments.

FIG. 5B illustrates the transduction layer of the energy harvestingmicrostructure of FIG. 5A, generally at 550, according to additionalembodiments of the invention. With reference to FIG. 5B, the secondconductor 508 is shown in a top view. The first conductors 512 a and 512b are shown as well as first conductors 552 a and 552 b from optionalthird and fourth transduction regions.

In the case where separate electrical isolation between transductionregions is desired, the second conductor is partitioned into an annularring around the transduction region. In such a case, with appropriatepatterning and etching 518 is an insulating material such as silicondioxide and annular rings 554, 556, 558, and 560 become the secondconductors. The conductors can be made from doped silicon, metal,LiNaO3, or other conductive film.

FIG. 5C illustrates, generally at 570, a plurality of transductionregions, according to additional embodiments of the invention. Withreference to FIG. 5C, a plurality of transduction regions are arrangedin series and are configured for d₃₃ poling. The series arrangement oftransduction regions has a first conductor 572 in electrical contactwith a first transduction region 574. The first transduction region 574is in electrical contact with the first conductor 572 and a secondconductor 576. The second conductor 576 is in electrical contact withthe first transduction region 574 and a second transduction region 578.The second transduction region 578 is in electrical contact with thesecond conductor 576 and a third conductor 580. The third conductor 580is in electrical contact with the second transduction region 578 and athird transduction region 582. The third transduction region 582 is inelectrical contact with third conductor 580 and a fourth conductor 584.

In one embodiment, the contribution from each transduction region isdependent on the surface area of each electrode. Contributions from eachof the transduction regions 574, 578, and 582 is obtained by poling eachsuccessive transduction region so that the harvesting circuit willobtain either a parallel or series circuit. Series addition oftransduction regions as shown in FIG. 5C will increase the voltage ofthe electrical energy generated from the transduction regions, andparallel will increase the current.

FIGS. 1A-5C above and the discussion directed to them describe variousmethods of creating a transduction micro-electro mechanical system(T-MEMS) using a buckled membrane. In various embodiments, a T-MEMS ismade with a reduced stiffness microstructure (RSM). The RSM is made fromtwo opposing buckled membranes with tailored properties.

Referring to FIGS. 6A, 6B, and 6C, FIG. 6A illustrates aforce-displacement characteristic of a buckled membrane, according toembodiments of the invention. A first buckled membrane is showngenerally at 600 in FIG. 6A. A first buckled membrane 602 has a bucklingdirection indicated at 603; the first buckled membrane 602 is part of afirst buckling layer 605. The first buckled membrane 602 is loaded at604, which causes the membrane to deflect opposite to the bucklingdirection 603. A force-displacement plot is shown at 606 with forceplotted on the vertical axis at 608 and displacement along a horizontalaxis at 610. In one embodiment, the force-displacement plot 606 is themeasurement shown earlier in FIG. 2C.

The load 604, applied to the first buckled membrane 602, is plotted at614, the load 614 produces a displacement 612 as indicated on curve 606.A point 616 indicates the zero slope of the force-displacement plot 606which is where the stiffness of the first buckled membrane 602 is zero.

Referring now to 630 in FIG. 6B, a second buckled membrane 632 ispositioned to oppose the first buckled membrane 602. The second buckledmembrane 632 is part of a second buckling layer 635. The second buckledmembrane 632 is designed to have a force-displacement characteristic 636that is identical to the force-displacement characteristic 606 of thefirst buckled membrane 602. As used in this detailed description ofembodiments, the term “opposing” refers to the situation where membraneshave buckled in the opposite direction. Such a case is illustrated bybuckling direction 633 (arrow to the right) which is opposite to thebuckling direction 603 (arrow to the left) of the first buckled membrane602 (FIG. 6A).

A load 634, imparted by the first buckled membrane 602, displaces thesecond buckled membrane 632 to the left. A point 646, on thedisplacement axis, indicates the zero slope of the force-displacementplot 636 which is where the stiffness of the second buckled membrane 632is zero.

Referring now to 660 in FIG. 6C, the first buckled membrane 602 and thesecond buckled membrane 632 have been brought together into contact witheach other, thereby developing a mutual preload, to the point where thepoint 616 and the point 646 overlap each other, resulting in point 662.Such a condition is referred to as aligning the force-displacementresponses or characteristics of the two opposing membranes. The force612 and 642 represent equal and opposite forces applied to buckledmembrane 602 and 632, respectively. Loads 612 and 642 are indicated in660 as coincident with each other.

FIG. 7A illustrates an operating zone of a reduced stiffnessmicrostructure (RSM), from 660 in FIG. 6C, according to embodiments ofthe invention. Referring now to FIGS. 6A, 6B, 6C, and FIG. 7A, theopposing buckled membranes were selected to provide very similar toidentical force-displacement responses. In this case, a zero-stiffnessoperating zone was desired, the aligned opposing buckled membranesprovide a zero-stiffness operating zone indicated at 702. In theoperating zone, a force applied to a membrane surface at 664 or 666,which is along the buckling directions 668 of the membranes, producesmotion of the buckled membrane microstructure.

Asymmetry in the force-displacement characteristics results in a netnon-zero force exerted between the membranes along the buckling axis intwo zones. These two zones function as potential energy stops whichprovide restorative forces that help keep the opposing buckled membranesin the operating zone 702. Two such non-zero force conditions areillustrated in FIG. 7A.

A first zone where the net force between the membranes is non-zero isindicated at a zone 704. In the zone 704, the second buckled membrane632 exerts a force on the first buckled membrane 602 which is largerthan the opposing force exerted by the first buckled membrane 602 (e.g.706 on the curve 636), thereby causing the opposing buckled membranes tomove to the right for displacement positions in the zone 704, such as712. At displacement 712, the net non-zero force exerted on the opposingbuckled membranes is the difference between the curve 636 and the curve606, which is indicated at 710. The force-displacement characteristics606 and 636 for the buckled membranes 602 and 632 diverge in the zone704 resulting in a potential energy stop at a first limit of thetranslation range 668 (FIG. 6C in 660). At displacement 712 the netnon-zero force 710 manifests on the opposing buckled membranes asindicated by an arrow 713 which provides a restorative force tending toreturn the opposing buckled membranes to the operating zone 702.

A second zone where the net force between the opposing buckled membranesis non-zero in indicated at 708, which is to the right of the operatingzone 702. In the zone 708, the first buckled membrane 602 exerts a forceon the second buckled membrane 632 which is larger than the opposingforce exerted by the second buckled membrane 632 (e.g., 716 on the curve606), thereby causing the opposing buckled membranes to move to the leftfor displacement positions in the zone 708, such as 714. At displacement714, the net non-zero force exerted on the opposing buckled membranes isthe difference between the curve 606 and the curve 636, which isindicated at 720. The force/displacement characteristics 606 and 636 forthe buckled membranes 602 and 632 diverge in the zone 708 resulting in asecond potential energy stop at a second limit of the translation range668 (FIG. 6C in 660). At displacement 714 the net non-zero force 720manifests on the opposing buckled membranes as indicated by an arrow 722which provides a restorative force tending to return the opposingbuckled membranes to the operating zone 702. Thus, a potential energywell is created with two opposing buckled membranes withforce-displacement characteristics that have been aligned.

The dynamic response to motion, described directly above, the case wherea zero-stiffness zone is created in a RSM can be referred to as apotential energy well. At one end of the translation range, along thebuckling directions or axes 668, a first potential energy stop existswithin the zone at 704. A second potential energy stop exists at theother end of the translation range within the zone 708. In thezero-stiffness zone 702, applied force results in kinetic energy ormotion and flexing of the buckled membranes along 668. The motion of thebuckled membranes along 668 (in the operating zone 702) results in adecrease in potential energy of one membrane with a correspondingsimultaneous increase in potential energy of the other membrane whilethe contact area of the membranes translates along the bucklingdirections or axes 668. Outside of the operating zone 702, the potentialenergy of one buckled membrane is larger than the potential energy ofthe other buckle membrane. This condition results in a potential energystop as previously described.

The alignment illustrated in FIG. 7A illustrates a method to achieve azero-stiffness elastic micro-mechanical structure (RSM). Variations fromzero are obtained in different ways, according to different embodiments.In one method, the membranes do not have identical force-displacementcharacteristics. This is accomplished by making one membrane stifferthan the other. Membrane stiffness is related to membrane thickness, aswell as to membrane composition, such as membrane area, materialproperties, etc. Therefore, in order to create membranes with differentforce-displacement characteristics, the membranes are intentionally madeto be non-identical to each other. One example of non-identicalforce-displacement characteristics is illustrated in FIG. 7B.

FIG. 7B illustrates, generally at 750, another operating zone of areduced stiffness microstructure, according to embodiments of theinvention. With reference to FIG. 7B, a single operating zone isillustrated for a RSM. A force-displacement curve 756 results from afirst buckled membrane. A second buckled membrane has aforce-displacement curve represented by 758. Force is plotted on an axis752 and displacement of the contact area is plotted on an axis 753. Bothbuckled membranes are in contact with each other and a state of preloadexists between the two buckled membranes, which causes the membranes'force-displacement characteristics 756 and 758 to become aligned.However, in the present case, there is always a non-zero force existingbetween the buckled membranes. The magnitude of the force is representedby the difference between force-displacement characteristics 756 and758. For example, at a displacement indicated at 754 the non-zero forceis indicated at 760. Note that the magnitude of 760 is smaller thaneither 762 or 764. Thus, a reduced stiffness microstructure results fromthe two opposing buckled membranes with non-identical mechanicalproperties.

In one or more embodiments, when the magnitude of 760 is constant acrossthe range of displacement a constant force micro-spring results. Mostlinear springs are described by the relationship F=kx, where F is theforce, k is the spring constant, and x is the displacement. Such aspring develops a force that is proportional to the displacement, whichmeans that as the displacement increases the force increases and viceversa. The spring represented in FIG. 7B is described by therelationship F=c. Where F is the force and c is a constant related tothe buckled stiffness of the pair of buckled membranes.

FIG. 8 illustrates, generally at 800, locating two substrates, accordingto embodiments of the invention. With reference to FIG. 8, in oneembodiment, the opposing buckled membranes previously discussed areconstructed on separate substrates and then the substrates are locatedvia micromanipulation before joining the substrates together. Buckledmembranes can be released from their respective substrates either beforejoining the substrates together or after the substrates are joinedtogether via the methods previously described or by methods as of yetunknown. Embodiments of the invention are not limited by the way inwhich a buckled membrane is released from the substrate.

In one embodiment, a first multilayered microstructure 802 has asubstrate indicated at 804 and a multi-layered buckling layer indicatedby layers 822, 806, and 823. In one embodiment, 822 is a layer ofsilicon dioxide followed by a conductive layer, 806 is a layer oftransduction material, and 804 is mono-silicon. The transduction layer806 is followed by a second conductive layer 823. A second multilayeredmicrostructure 808 has a substrate indicated at 810 and a multilayeredbuckling layer 820, 812, and 813. In one embodiment, 810 is amono-silicon layer, 820 is a layer of silicon dioxide followed by anoptional conductive layer and an optional layer of transduction material812. The optional transduction layer 812 is followed by an optionalsecond conductive layer 813. In one embodiment, spacers 814 and 816 aremade from poly-silicon and are deposited on 813. A thickness 818 of thespacers is selected to provide the proper initial displacement of thebuckled membranes following release of the membranes from theirsubstrates. In the examples given in conjunction with FIGS. 6A, 6B, 6C,and FIG. 7A, the desired initial displacement of the opposing buckledmembranes is indicated as 616 and 646 (616 is equal to 646 in theexample of FIGS. 6A and 6B). Therefore, the thickness 818 of the spacer814 and 816 is two times the displacement 616. Alternatively, thethickness of the spacers 816/814 can be selected to provide a differentforce-displacement characteristic for the opposing coupled buckledmembranes according to the teaching presented herein.

To provide the desired distance between buckled membranes, the spacers816 and 814 can be chemically mechanically polished to planarize thedeposited surface. Alternatively, optional spacers 832 and 834 can bedeposited onto the layer 806. In such a case, the thickness 818 of thespacer 816 and 814 combined with the thickness 836, of the spacers 832and 834, to equal two times the displacement 616 (which is alsoequivalent to the displacement 646 or 662). For example, the thickness818=836=616. Alternatively, when two spacers are used, the desiredseparation between the top surfaces of the opposing buckling layers canbe apportioned between the two spacers by any fractional distribution.

The substrate 808 and the substrate 802 are located relative to oneanother by micromanipulation, indicated by arrows 826 and 830 and thenthe substrates are brought into contact with one another as indicatedwith arrows 824 and 828. The substrates are joined together by variousmeans such as direct bonding, Plasma Activated bonding, Eutecticbonding, etc. Those of skill in the art will recognize other bondingmethods that are suitable for this use. The bonding methods listedherein are given by way of example and do not limit embodiments of theinvention.

FIG. 9 illustrates, generally at 900, bonding two substrates together,according to embodiments of the invention. With reference to FIG. 9, thetwo substrates described in FIG. 8 are shown bonded together in 900.Note that in FIG. 9, the membranes have not yet been released from thesubstrates. Following bonding of the substrates, buckled membranes arereleased from the substrate 808 and 802. In one embodiment, the outersurface of 808 and 802 are patterned and etched to remove substrate 804and 810 from the region between the spacers. The resultingmicrostructure is shown in FIG. 10A.

FIG. 10A illustrates the state of the buckled membranes after themembranes have been release from the substrates. Alternatively, in someembodiments, the membranes are released before the substrates are joinedtogether. In such a case the membranes are released before thesubstrates are joined together and care is taken to align buckledmembranes from one substrate with buckled membranes from the othersubstrate. Membranes are released following any of the methods known tothose of skill in the art. The example given above is for the purpose ofillustration and does not limit embodiments of the invention in any way.

FIG. 10A illustrates, generally at 1000, a pair of coupled buckledmembranes, according to embodiments of the invention. With reference toFIG. 10A, a multi-layered microstructure 1002 has a substrate 1004 and afirst multi-layered buckling layer, which includes a sub-layer 1022, asub-layer 1006, and a sub-layer 1021. A first buckled membrane 1028 hasbeen released by removal of the substrate 1024 a over an area 1023.

A multi-layered microstructure 1008 has a substrate 1010 and a secondmulti-layered buckling layer, which includes a sub-layer 1012, asub-layer 1019, and a sub-layer 1020. A second buckled membrane 1026 hasbeen released by removal of the substrate 1024 b over an area 1023.Removal of substrate 1024 a and 1024 b cause the first buckled membrane1028 to contact the second buckled membrane 1026 over a contact area1030.

A thickness 1034 of spacers 1014 and 1016 has been designed to providethe desired force-displacement characteristic for the reduced stiffnessmicrostructure (RSM). Spacers 1014 and 1016 can be sized to provide anopen area (void) between the buckled membranes that is not the same sizeas the area indicated at 1023. Such variation is indicated by dashedlines 1031 and 1032.

In one or more embodiments, the RSM of FIG. 10A has been designedaccording to the descriptions presented in support of the previousfigures to produce a zero-stiffness RSM or a reduced stiffness RSM. Inone or more embodiments, the RSM of FIG. 10A is incorporated into aT-MEMS thereby providing buckled membranes that have a transductionlayer or region with appropriate electrical contacts, from whichelectrical energy is harvested. In various embodiments, a transductivelayer(s) or region(s) can be present in one or both of the membranes. Asdescribed above in conjunction with the preceding figures, vibrationfrom a variety of sources will flex the buckled membranes within theT-MEMS of FIG. 10A. A sprung mass 1036 will enhance the flexing of thebuckled membranes. The buckled membranes flex within a potential energywell created by the RSM structure. The RSM structure overcomes theinitial stiffness of the spring/transduction material making it possibleto cause the transduction material to flex for much lower levels ofapplied vibration or by using a smaller mass for the same level ofapplied vibration.

As a point of comparison with the single buckled membrane of FIG. 2C,2000 μN of force is needed to move the single buckled membrane from itsrest position to approximately 9 μm displacement indicated at 246. Inone embodiment, if the difference in stiffness between the two buckledmembranes used in FIG. 10A is kept to 5% and the buckled membranes havethe same nominal stiffness/displacement characteristic as that shown inFIG. 2C, then the stiffness of the dual buckled membrane RSM of FIG. 10Ais only 5% as great as the stiffness of the single buckled membraneshown in FIG. 2C. This means that it would only take 100 μN of force toproduce an equivalent acceleration of the dual buckled membrane RSM.Thus, a sprung mass only 1/20 as large (0.01 gm) as the mass used in theexample of FIG. 2C (0.2 gm) above can be used to produce the sameacceleration while straining twice as much transduction material (thisresult is obtained when two similar membranes are being flexedsimultaneously). Note that more than twice as much transduction materialcan be flexed with a RSM structure as taught herein. A RSM structure canbe designed to compensate and effectively zero out the mechanicalstiffness of the membranes. Thereby permitting a large amounttransductive material to undergo flexing and subsequent contribution toenergy harvesting.

The buckled membranes used in the RSM of FIG. 10A can be made withtransduction layers (regions) that have been poled in either d₃₁ or d₃₃orientation. Note also that a pair of buckled membranes used in a RSMcan be oppositely poled (for given poling direction) to cause thecontributions from each membrane to add in series. For example, if twod₃₃ poled transduction microstructures are mated together to form a RSMa first microstructure is poled such that the first conductor 512 a(FIG. 5A) is at “+” polarity and the mating microstructure is then poledso that the first conductor 512 a (FIG. 5A) would be at “−” polarity.

FIG. 10B illustrates, generally at 1040, a method for harvesting energyfrom buckled membranes according to one embodiment of the invention.With reference to FIG. 10B, a method starts at a block 1042. At a block1044 a first potential energy stop is provided. At a block 1046 a secondpotential energy stop is provided. Energy is harvested at a block 1048as a pair of buckled membranes flexes between the first potential energystop and the second potential energy stop. The method ends at a block1050.

FIG. 10C illustrates, generally at 1060, charging a battery with energyharvested from buckled membranes according to one embodiment of theinvention. With reference to FIG. 10C, a T-MEMS structure is shown at1000. The T-MEMS structure 1000 is connected an interface circuit 1064by electrical connection 1062. Electrical connections 1062 are connectedto a first conductor and a second conductor associated with thetransduction material of the T-MEMS 1000 as described above inconjunction with the preceding figures. The interface circuit 1064 iselectrically connected at 1066 to a rechargeable power source 1068. Therechargeable power source 1068 can be a battery, a capacitor, or anotherstructure configured to store electrical energy. In various embodiments,the rechargeable power source 1068 can be a rechargeable battery madewith various chemistry such as Lithium Ion, Lithium Polymer, LithiumIron Phosphate, etc. or with battery chemistry that has not yet beeninvented. Embodiments of the invention are not limited by the type ofbattery chemistry employed for the rechargeable power source 1068.

The interface circuit 1064 has electrical parameters, such as loadresistance, etc. that are selected to optimize the amount of electricalpower that is transferred from T-MEMS 1000 to the rechargeable powersource 1068. The interface circuit 1064 can be configured to control thecharging of the rechargeable electrical power source 1068. Rechargeablebatteries typically have minimum and maximum voltage thresholds that thecharging must occur within, thus the interface circuit 1064 can beconfigured to perform the charge controller function as well.

In various embodiments, a plurality of T-MEMS devices are used in placeof the T-MEMS 1000 shown in FIG. 10C. A plurality of T-MEMS devices canbe configured both in a single plane and in multiple planes. In one ormore embodiments, T-MEMS devices are configured in mutually orthogonalplanes, in other embodiments, T-MEMS devices are configured in planesthat make an angle less than ninety degrees with one another. Byconfiguring T-MEMS devices in different planes the energy harvestingdevice can respond to vibrations independent from the orientation of theenergy harvesting device to the vibration excitation field.

FIG. 10D illustrates, generally at 1080, a method for harvesting energyfrom a buckled membrane according to one embodiment of the invention.With reference to FIG. 10D, a method starts at a block 1082. At a block1084 a buckled membrane is flexed with an attached mass. At a block 1086energy is harvested from the buckled membrane. The method stops at ablock 1088. One or more T-MEMS are used in the block 1084. The T-MEMSused in the block 1084 can be configured with single buckled membranes,dual buckled membranes configured as RSM or a plurality of singlebuckled membranes or a plurality of dual buckled membranes configured asRSM devices. In various embodiments, the energy harvesting at 1086 canbe implemented with an interface circuit as shown in FIG. 10C.

FIG. 11 illustrates, generally at 1100, a mass (or spacer) locatedbetween a pair of coupled buckled membranes, according to embodiments ofthe invention. With reference to FIG. 11, a mass 1106 is deposited onone of the buckling layers (1028 or 1026) before the substrates arealigned and bonded together, such as in the state of assembly shown inFIG. 8. The thickness of the spacers 1102 and 1104 is sized toaccommodate the added thickness of the mass 1106, consistent with thedesired elastic response of the RSM. For example, if a mass 1106 wereadded to the RSM of FIG. 10A, an added thickness 1108 is added to thethickness 1034 of the spacers 1014 and 1016 in order to preserve theelastic response of the RSM.

The potential energy well described above in conjunction with a RSM thathas a zero-stiffness zone provides potential energy boundaries whichserve to limit the displacement of a pair of opposing coupled buckledmembranes. If excitation energy is sufficiently high, the pair ofcoupled buckled membranes can travel outside of the potential energywell boundaries and “pop through” to one side or the other. Poppingthrough can be prevented with mechanical stops. FIG. 12A and FIG. 12Billustrate mechanical stops used with a pair of coupled buckledmembranes, according to embodiments of the invention.

With reference to FIG. 12A, a RSM is shown at 1201. In one embodiment,1201 is the RSM 1000 from FIG. 10A. An upper layer 1202 has been micromachined to produce a stop 1204 which has a height or thickness 1205. Alower layer 1206 has been micro machined to produce a stop 1208, whichhas a height 1205.

In FIG. 12B at 1250 the upper layer 1202 and the lower layer 1206 havebeen bonded to the RSM 1201. The upper layer 1202 and the lower layer1206 are positioned through micromanipulation to place the stops at adesired location. Desired locations can be any position behind thebuckled membrane. For example, the stop 1204 or stops 1204/1208 can becentered on the buckled membrane's area, off to one side, etc. The stopcan be arrayed as a single stop (as shown in FIG. 12A and FIG. 12B) orthe stop 1204 can be a plurality of stops.

The stops 1204 and 1208 limit the travel of the coupled buckledmembranes to the distance shown at 1258 and 1260. A measurement of theforce-displacement characteristic of a buckled membrane will provide theinformation needed to determine the distance represented by 1258 and1260. Referring to FIG. 7A, one design point can be to set 1258 equal toone half of the width of the operating zone 702. Another design pointcan be wider, which can include a portion of zone 704 and 708, keepingin mind the desire to prevent pop through. Alternatively, the stops canbe located to allow pop through.

A sprung mass can be added to the buckled membranes of FIG. 12A and FIG.12B to enhance flexing of the buckled membranes and therefore theproduction of energy for harvesting when configured as a T-MEMS.

Alternatively, in one or more embodiments, the T-MEMS shown in thefigures above can be configured with an activation circuit. In someembodiments the membranes are electrically independent from each othercan be configured for individual control. In such a case, the activationcircuit causes one membrane to become strained which will thereby causethe first cycle of vibration to occur. Once placed in motion, due to thezero stiffness or near zero stiffness of the RSM structure, severalcycles of vibration will occur; energy is harvested during thesevibration cycles. The activation circuit is configured to providesubsequent activation of one or the other of the membranes in order toovercome internal friction if necessary.

FIG. 13A illustrates energy harvesting from a pair of coupled buckledmembranes. With reference to FIG. 13A, 1300 illustrates load deflectioncurves and displaced shape for a pair of buckled membranes. A firstbuckled membrane 1318 a is illustrated in contact with a second buckledmembrane 1320 a, both of which are in contact at a point “C,” labeled as1324.

A series of force displacement plots for these buckled membranes isshown at 1302, 1304, 1306, where the magnitude of the force required todisplace the first buckled membrane 1318 a in the negative direction (tothe left) is indicated by the vertical axis at 1306. The force requiredto displace the second buckled membrane 1320 a in the positive direction(to the right) is indicated by the vertical axis at 1302, displacementalong the buckling axes is indicated along a horizontal axis at 1304.

In one embodiment, the second buckled membrane 1320 a contains atransductive layer with first and second electrodes. The first buckledmembrane 1318 a does not contain a transductive layer; however in otherembodiments the first buckled membrane 1318 a does contain atransductive layer. The force displacement curve 1308 corresponds to thesecond buckled membrane 1320 a when the transductive layer is in an“open” state, i.e., not connected to an energy harvesting circuit orother load that would allow the electrical charge to dissipate. Theforce displacement curve 1312 corresponds to the first buckled membrane1318 a. Note that by design of the microstructure, the peak value offorce on curve 1308 is greater than the peak value of the force on curve1312. As is known to those of skill in the art, when a transductionlayer is connected to a circuit such that electrical charge can flowfrom the transduction layer, the stiffness of the transduction layer isreduced. This reduction in stiffness results in a reduction of the forceneeded to produce a given displacement of the second buckled membrane1320 a. Thus, in the figures of 1330, a force displacement curve 1332corresponds to the “shorted” condition for the transduction layer of thesecond buckled membrane 1320 a. The magnitude of the force on curve 1332is lower than the magnitude of the force on curve 1308 (1300). Thedesign condition illustrated in FIG. 13A between the first and thesecond buckled membranes is 1308 (open)>1312>1332 (shorted) within anoperating range of the microstructure, such as the operating range 1362shown in 1360.

In one embodiment, the buckled membranes 1318 a and 1320 a are usedtogether with a control circuit to harvest energy. Description of anenergy harvesting/control circuit is given below in conjunction withFIG. 14. Both buckled membranes are in a rest position as indicated atposition “C” labeled as 1324 in 1300. The transductive layer of thesecond buckled membrane 1320 a is in a state of compression while atrest. An electrical potential is applied to the transductive layer ofthe second buckled membrane 1320 a by a control circuit, which causesthe state of compression within the second buckled membrane 1320 a todiminish, thereby resulting in a displacement of the membranes to theright coming to rest at a position “A” indicated at 1326 and “A” at 1316at 1300. Note, that in the embodiment described herein, one of skill inthe art will recognize that the electrical potential applied by thecontrol circuit is causing the second buckled membrane 1320 a tocontract. The displaced shapes of the buckled membranes are indicated bythe dashed lines 1318 b and 1320 b.

The electrical potential is removed from the second buckled membranecausing the buckled membranes to flex back to the position indicated by“C” at 1314 and 1324 within 1330.

The charge harvesting circuit is engaged which permits electrical energyto be harvested from the transduction layer of the second buckledmembrane 1320 a/1320 b. The stiffness of the second buckled membrane1320 a/1320 b drops when the harvesting circuit is engaged, thus theforce displacement profile of the second buckled membrane 1320 a/1320 bdrops to 1332 essentially moving the state of force between the twobuckled membranes from point “C” indicated at 1314 in 1330 to point “D”indicated at 1334 in 1330. This condition presents an unbalanced forcerelationship since force exerted by the first buckled membrane 1318a/1318 b (rightward acting force) is greater than the force exerted bythe second buckled membrane 1320 a/1320 b (leftward acting force), thusmotion to the right results. Charge is harvested during the resultingmotion of the buckled membranes from “D” to “E” in 1330.

With reference to 1360, in one embodiment, an operating zone for thebuckled membranes is indicated by 1362. The contact point between thetwo buckled membranes moves along the buckling axes within the operatingzone. Those of skill in the art will recognize that the physicalproperties of the buckled membranes and the steps described that harvestenergy can be changed and/or adapted from those shown and described withthe diagrams herein. However, such changes and or adaptations are withinthe scope of the teaching presented herein which uses the variablestiffness of the buckled membrane to facilitate energy harvestingtherefrom.

FIG. 13B illustrates energy harvesting from a pair of coupled buckledmembranes, according to additional embodiments of the invention. Withreference to FIG. 13B, 1370 illustrates load deflection curves anddisplaced shape for a pair of buckled membranes. In the embodimentsrepresented in FIG. 13B the load deflection curve 1308 (associated withthe second membrane 1320 a/1320 b) and a load deflection curve 1374(associated with the first membrane 1318 a/1318 b) do not intersect atpoint C (1314).

Translation of the contact area of the pair of buckled membranes isconstrained to an operating range 1376 by a stop 1378 and a stop 1380.Stops, such as 1378/1380 can also be used with the buckled membranesdescribed above in FIG. 13A. Similarly, buckled membranes havingdifferent load deflection curves, such as for example 1372, 1374 can beused to harvest energy.

Another embodiment of load deflection curves for a pair of buckledmembranes is illustrated by 1390. In 1390, a first buckled membrane suchas 1318 a/1318 b has a load deflection curve represented by 1392. Loaddeflection curve 1392 exceeds load deflection curve 1308 over a portionof the x axis 1304 displacement range. In one embodiment, an operatingrange of the resulting coupled buckled membrane structure is indicatedat 1394.

The places of intersection between the load deflection curves 1392 and1308, such as point C (1314) in 1390, can serve as “stops” for thetranslation of the contact area of the coupled buckled membranes. Suchlocations of crossing e.g., point C (1314) 1390 are characterized byforces of opposite sign. To the left of point C (1314) 1390 the firstbuckled membrane 1318 a/1318 b exerts the greater force acting to theright. To the right of point C (1314) 1390 the second buckled membraneexerts the greater force acting to the left. The translational motion ofthe coupled buckled membranes can be halted by such an arrangement ofload deflection curves, thereby eliminating the need for a stop such asthe stop shown in 1370.

Thus, a variety of different load deflection curves associated withbuckled membranes can be used within the teachings of the embodiments ofthe invention to harvest energy. The load deflection curves providedherein are merely illustrations of examples and do not limit theembodiments of the invention.

FIG. 14 illustrates, generally at 1400, a system for harvesting energy,which includes an activation/control circuit. With reference to FIG. 14,a microstructure that contains a pair of buckled membranes is indicatedat 1402. A first buckled membrane 1404 and a second buckled membrane1406 are in contact with each other and the mutual contact area flexesalong buckling axes 1412. In one embodiment, the second buckled membrane1406 includes a transductive layer with inner and out electrodes and isindicated at 1410. Other layers of the second buckled membrane areindicated at 1408.

A harvesting/control circuit 1420 is electrically connected toconductors associated with the transduction layer 1410. In variousembodiments, the harvesting/control circuit 1420 is used to bothenergize the transduction layer and to collect electrical energy fromthe transduction layer of a buckled membrane such as the second buckledmembrane 1406. Energy collected with the harvesting circuit is indicatedat 1430. In some embodiments, the harvesting/control circuit 1420 isconfigured to harvest energy. In other embodiments, theharvesting/control circuit 1420 is configured to both control variousaspects of the buckled membrane and to harvest energy.

FIG. 15 illustrates, generally at 1500, a method for harvesting energywith buckled membranes. With reference to FIG. 15, a process starts at ablock 1502. At a block 1504 a transduction layer within a buckledmembrane is charged while buckled membrane flexes. At a block 1506energy is harvested from the buckled membrane. At a block 1508 theprocess ends.

The T-MEMS device is also configured with an appropriate circuit inorder to maximize energy transfer to a storage device such as arechargeable battery, one example of this was illustrates above in FIG.10C. In other embodiments, the appropriate circuits can be included onthe substrate used for the T-MEMS.

Thus, in various embodiments, T-MEMS devices provide structures thatharvest energy that would otherwise be wasted. T-MEMS devices can beused to power a wide range of devices. A non-limiting list of suchdevices includes, but is not limited to, implantable devices, e.g., pacemakers, hearing aids, etc.; handheld devices, e.g., telephones, smartphones, tablets, laptop computers, global positioning system (GPS); andlarge devices that heat and cool buildings and power automobiles,trucks, busses, building, etc.

RSM devices incorporating coupled buckled membranes can be made from asingle substrate in order to avoid micromanipulation of separatesubstrates. A pair of opposing buckled membranes needs a cavity intowhich the membranes buckle into and then contact each other followingrelease. Such cavities can be created by etching into a multilayeredmicrostructure, such as one made from a series of layers of silicon oninsulator (SOI). Following creation of a cavity, membranes can bereleased by etching each respective surface or by ion bombardment on therespective surfaces.

For purposes of discussing and understanding the embodiments of theinvention, it is to be understood that various terms are used by thoseknowledgeable in the art to describe techniques and approaches.Furthermore, in the description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be evident, however, toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In some instances, well-knownstructures and devices are shown in block diagram form, rather than indetail, in order to avoid obscuring the present invention. Theseembodiments are described in sufficient detail to enable those ofordinary skill in the art to practice the invention, and it is to beunderstood that other embodiments may be utilized and that logical,mechanical, electrical, and other changes may be made without departingfrom the scope of the present invention.

As used in this description, “one embodiment” or “an embodiment” orsimilar phrases means that the feature(s) being described are includedin at least one embodiment of the invention. References to “oneembodiment” in this description do not necessarily refer to the sameembodiment; however, neither are such embodiments mutually exclusive.Nor does “one embodiment” imply that there is but a single embodiment ofthe invention. For example, a feature, structure, act, etc. described in“one embodiment” may also be included in other embodiments. Thus, theinvention may include a variety of combinations and/or integrations ofthe embodiments described herein.

While the invention has been described in terms of several embodiments,those of skill in the art will recognize that the invention is notlimited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

What is claimed is:
 1. An energy harvesting microstructure, comprising:a substrate, a first buckled membrane, the first buckled membrane has abuckling axis and is connected to the substrate, the first buckledmembrane further comprising: a transduction material, a first conductor,the first conductor is applied to the transduction material; and asecond conductor, the second conductor is applied to the transductionmaterial, wherein electrical charge is harvested from the transductionmaterial when the first buckled membrane is translated along thebuckling axis.
 2. The microstructure of claim 1, further comprising: amass, the mass is mechanically coupled to move with the transductionmaterial.
 3. The microstructure of claim 1, wherein the transductionmaterial is selected from the group consisting of PVDF, Polyimide, PZT,AlN, ZnO and a user defined material.
 4. The microstructure of claim 1,wherein the transduction material is poled within a plane of the firstbuckled membrane.
 5. The microstructure of claim 4, wherein the firstconductor and the second conductor extend along a thickness of thetransduction material.
 6. The microstructure of claim 4, wherein thesecond conductor is in contact with a second transduction material. 7.The microstructure of claim 4, wherein the first conductor is locatedwithin the transduction material.
 8. The microstructure of claim 1,wherein the transduction material is poled perpendicular to a plane ofthe first buckled membrane.
 9. The microstructure of claim 8, whereinthe first conductor and the second conductor extend along a length ofthe transduction material.
 10. The microstructure of claim 9, whereinthe transduction material is in the shape of a layer.
 11. Amicrostructure for harvesting energy, comprising: a first substrate; afirst buckled membrane, the first buckled membrane is connected to thefirst substrate, the first buckled membrane has a first transductionmaterial; a second substrate; and a second buckled membrane, the secondbuckled membrane is connected to the second substrate; the first buckledmembrane and the second buckled membrane are in contact over a contactarea electrical charge is available for harvesting in the firsttransduction material when the contact area of the buckled membranestranslates along one of their mutually opposite buckling directions. 12.The microstructure of claim 11, wherein the second buckled membrane hasa second transduction material, the contact area is sized to createsubstantially a zero stiffness when the buckled membranes are flexedalong one of their mutually opposite buckling directions, and electricalcharge is available for harvesting in at least one of the firsttransduction material and the second transduction material.
 13. Themicrostructure of claim 12, further comprising; a mass, the mass ismechanically coupled to move with the contact area.
 14. Themicrostructure of claim 11, wherein the first transduction material ispoled using a first conductor and a second conductor, the firsttransduction material is poled within a plane of the first buckledmembrane.
 15. The microstructure of claim 12, wherein the secondtransduction material is poled using a first conductor and a secondconductor, the second transduction material is poled within a plane ofthe second buckled membrane.
 16. The microstructure of claim 14, whereinthe first conductor is located within the first transduction material.17. The microstructure of claim 15, wherein the first conductor islocated within the second transduction material.
 18. The microstructureof claim 11, the first transduction material is poled perpendicular to aplane of the first buckled membrane.
 19. The microstructure of claim 12,wherein the second transduction material is poled perpendicular to aplane of the second buckled membrane.
 20. The microstructure of claim18, wherein the first conductor and the second conductor extend along alength of the transduction material.
 21. The microstructure of claim 20,wherein the transduction material is in the shape of a layer.
 22. Themicrostructure of claim 12, wherein energy is harvested from the firsttransduction material while energy is not harvested from the secondtransduction material.
 23. The microstructure of claim 12, whereinenergy is harvested from the first transduction material and the secondtransduction material.
 24. The microstructure of claim 23, wherein thefirst transduction material and the second transduction materialtogether form a single electrical circuit.
 25. The microstructure ofclaim 23, wherein the first buckled Membrane further comprises anadditional transduction material.
 26. The microstructure of claim 25,wherein the first transduction material occupies a larger volume thanthe additional transduction material occupies.
 27. The microstructure ofclaim 11, wherein the first transduction material is selected from thegroup consisting of PVDF, Polyimide, PZT AlN, ZnO and a user definedmaterial.
 28. The microstructure of claim 12, wherein the secondtransduction material is selected from the group consisting of PVDF,Polyimide, PZT, AlN, ZnO and a user defined material.
 29. A method toharvest energy with a microstructure, comprising: providing a firstpotential energy stop; and providing a second potential energy stop,wherein the first potential energy stop and the second potential energystop are separated by a distance, such that a pair of opposing buckledmembranes can flex between the first potential energy stop and thesecond potential energy stop as a contact area of the opposing buckledmembranes translates along a buckling axis, the opposing buckledmembranes are formed with a first buckled membrane and a second buckledmembrane; and harvesting energy from at least one layer of the opposingbuckled membranes when the contact area of the opposing buckledmembranes translate along the buckling axis.
 30. The method of claim 29,wherein the harvesting is from a transduction material of one of theopposing buckled membranes while energy is not harvested from atransduction material of the other of the opposing buckled membranes.31. The method of claim 29, wherein the harvesting is from atransduction material in the first buckled membrane and a transductionmaterial in the second buckled membrane.
 32. The method of claim 31,wherein the opposing buckled membranes together form a single electricalcircuit.
 33. The method of claim 29, further comprising; exerting anexcitation force on the contact area, wherein a mass is constrained tomove with the contact area and acceleration of the mass imparts theexcitation force.
 34. The method of claim 29, further comprising:charging a rechargeable power source with the energy from theharvesting.
 35. The method of claim 34, further comprising: providing anelectrical interface between the rechargeable power source and thetransduction material.
 36. A method for harvesting energy with amicrostructure, comprising: flexing a buckled membrane with a mass,wherein the buckled membrane is attached to the microstructure, thebuckled membrane has a transduction material, and the mass is coupled tothe transduction material; and harvesting energy from the transductionmaterial during the flexing.
 37. The method of claim 36, furthercomprising; summing energy from a plurality of buckled membranes,wherein the plurality of buckled membranes are attached to themicrostructure and each buckled membrane of the plurality of buckledmembranes contains at least one transduction material.
 38. The method ofclaim 36, wherein the buckled membrane contains more than onetransduction material.
 39. The method of claim 36, further comprising:charging a rechargeable power source with the energy from theharvesting.
 40. A method for harvesting energy from a microstructure,comprising: generating an electrical charge in a transduction layer, thetransduction layer is part of a first buckled membrane, the firstbuckled membrane has a buckling axis, wherein the generating occurswhile the transduction layer flexes; and harvesting the electricalcharge from the transduction layer when the first buckled membrane istranslated along the buckling axis.
 41. The method of claim 40, furthercomprising: providing a restoring force from a second buckled membrane,wherein the restoring force acts on the first buckled membrane over acontact area between the first buckled membrane and the second buckledmembrane.
 42. The method of claim 40, further comprising: controlling astiffness of the transduction layer during the harvesting.
 43. Themethod of claim 42, wherein a harvesting/control circuit is used duringthe controlling.
 44. The method of claim 42, wherein aharvesting/control circuit is used during the harvesting and thecontrolling.
 45. The method of claim 40, wherein a harvesting/controlcircuit is used during the harvesting.
 46. The method of claim 45,wherein the harvesting/control circuit can present at least twodifferent load conditions to the transduction layer.
 47. The method ofclaim 45, wherein a load condition is selected from the group consistingof open, short, in between open and short.